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The index is a whole number that ranges between 0 and 20 in France and up to above 30 in Canada. It is computed from five components. The first three components are numeric ratings of the moisture content of litter and other fine fuels, the average moisture content of loosely compacted organic layers of moderate depth, and the average moisture content of deep, compact organic layers. The last two components are the rate of fire spread if fuel is available for combustion, and the frontal fire intensity.[2]

The essential informations needed to calculate this index are :

the humidity of the air at the beginning of the afternoon (when it has its lowest value);

the temperature in the middle of the afternoon (when it has its highest value);

This index is computed for 40 zones of France, three are in the Landes (département with the biggest forest of France). Météo France sends the IFM to civil defense authorities, which allows the prepostioning of men and vehicles, especially air units. The computed index is revised every year to take new data into account.[4]

Each provincial forest fire prevention authority is calculating the FWI for the different regions under their jurisdiction. The Daily Severity Rating (DSR) is a numeric rating of the difficulty of controlling fires, based on the Fire Weather Index, that is calculated afterward. It reflects more accurately the expected efforts required for fire suppression.

A daily fire weather outlook is issued twice a day to alert the fire authority and help local firefighters to get organized. Provided informations are based on FWI values computed at 20 weather stations representative of the climatic diversity of the country.[5]